I think DP and his ex have a great co-parenting relationship. I am also a child of separated parents so am looking at it from that perspective too.
I think it is due to the following:
DP and his ex live within walking distance of each other and are committed to doing this until the children have grown up. This means that the children can maintain their social lives and clubs etc. no matter which house they are at.
They are flexible with contact. It works our the DSCs, ours and their mum's plans. If their mum wants a night out/ is busy with work / just fancies some time to herself the DSC are welcome here. Similarly, if, for some reason we are unable to have the children as usual she is flexible. We tell each other before booking holidays or if we are going to a wedding or something so that other parent can ensure that they are free. Also, if the DSCs fancy some more time here or with their mum then it's not an issue.
DP has never bad-mouthed their mother in front of them and, as far as I am aware, she hasn't done that to him.
Their mum has supported the children in getting to know me and forming a bond. This means that they are comfortable coming here and feel like they can be close to me without feeling disloyal to their mum.
Neither parent makes unilateral decisions about the children without discussion.